1. Field of the Invention
The present invention relates to quality evaluation and control of semiconductor multiple layer structures.
2. Background Art
An epitaxial wafer having a multiple layer structure is used for fabricating semiconductor devices, and the quality evaluation and control therefor are necessary in order to decrease the number of defectives and to reduce the fabrication cost. Items assessed for quality on an epitaxial wafer include thicknesses and compositions of the multiple layers. The thickness is mainly measured with a technique such as infrared-ray or X-ray interference by using interference of electromagnetic waves, or. ellipsometry (polarization analysis) by using change in the degree of polarization between incident and reflecting lights. On the other hand, the compositions of the layers in an epitaxial wafer are evaluated with band gap energies obtained by spectrum measurement of photoluminescence generated by irradiation of excitation light such as laser beams (for example, Masashi Ozaki, Semiconductor Evaluation Technique, Ch. 7 [in Japanese], ed. Takashi Katouda, Sangyo Tosho, Tokyo, 1989).
There are many items assessed on an epitaxial wafer besides the above-mentioned thickness and composition. One of them is the internal electric field strength, known to depend on the doping concentration and the state at the surface/interface in an epitaxial wafer. Further, the internal electric field strength was recently reported to have a correlation with current gain in a heterojunction bipolar transistor. Therefore, the internal electric field strength is regarded as an important item in quality evaluation on an epitaxial wafer.
The internal electric field strength can be measured with a technique called light modulated reflection spectroscopy or photoreflectance (hereinafter referred also to PR) spectroscopy. In this method, a change in reflectance, ΔR/R, caused by irradiating a sample with external modulated light such as laser light, called pumping light, is measured with light called probe light (detection light). The change in reflectance, ΔR/R, is recorded as a function of wavelength or photon energy of the probe light. In a sample having an internal electric field, the change in reflectance, ΔR/R, exhibits an oscillatory profile called Franz-Keldysh oscillations (for example, refer to “Franz-Keldysh Oscillations in Modulation Spectroscopy”, H. Shen and M. Dutta, Journal of Applied Physics, vol. 78, pp. 2151–2176 (1995)). The period of the oscillation is determined by the internal electric field strength. Therefore, the internal electric field can be investigated by analyzing the period of the Franz-Keldysh oscillations. Further, Japanese Patent laid open Publication 11-51856/1999 shows that the photoreflectance from a sample can be detected solely from the integrated intensity thereof, without a spectrometer.
The above-mentioned three items, thickness, band gap energy and internal electric field, are measured with techniques different from each other in the evaluation of the quality of the epitaxial wafer. Therefore, much time and work are needed in order to measure all the three items. Further, the cost is high for providing apparatuses for the three measuring techniques. This leads to a barrier for strict quality control.